LIMA, July 4 (Reuters) - One person died and police detainedone of the leaders of protests against Newmont's $5 billion goldmine in Peru on Wednesday, the day after clashes between policeand protesters left three people dead and 21 injured.

The second day of violence erupted in the northern region ofCajamarca as President Ollanta Humala came under criticism forsuspending freedom of assembly in the area late on Tuesday.

"One death has been confirmed, a civilian," Prime MinisterOscar Valdes told a press conference.

Left-wing leader Marco Arana, a soft-spoken former RomanCatholic priest who has rallied demonstrators to stopconstruction of the biggest mine in Peruvian history, saidpolice had beaten him and local TV showed photographs ofauthorities taking him away.

"They detained me and beat me a lot, inside the policestation they beat me again - punches in the face, the kidneysand insults," Arana said via Twitter.

Arana is widely thought to have presidential ambitions. Heand his allies on the left say Humala has drifted too far to theright since taking office and has put the interests of globalminers ahead of poor peasants.

Humala, a former military officer, says the mine proposed bythe U.S.-based Newmont would generate thousands of jobsand huge tax revenues. Protesters say it would cause pollution,contaminate water supplies and fail to bring local economicbenefits.

Opposition lawmakers and human rights groups denouncedArana's detention as part of a harsh crackdown by Humala - whotook office a year ago urging mediation to solve hundreds ofdisputes nationwide over natural resources but has been quick tosuspend civil liberties to quell protests.

Humala has implemented emergency measures involving thesuspension of civil liberties at least three times in the pastyear. He also arrested a mayor in the southern region of Cuscoin May for leading a protest against global miner Xstrata.

"What we have here is a repressive attitude, which isviolating the rule of law, and an intelligence service that isworking for the mining sector," said Congressman JorgeRimarachin, who once supported Humala but has defected from hisparty. "This is totally unacceptable."

Members of Humala's party blamed Arana and another stridentopponent of the mine - Gregorio Santos, the president ofCajamarca region - for inciting violence at a rally of 2,000people on Tuesday where protesters threw rocks and vandalizedpublic buildings.

"The only person responsible for this is Mr. Santos," saidl awmaker T eofilo Gamarra of Humala's Gana Peru party.

BUILDING RESERVOIRS

Official data shows at least 14 people have died duringHumala's term in protests over natural resources, compared with174 who were killed in similar circumstances from 2006 to 2011on the watch of his predecessor, Alan Garcia.

Newmont's project in Cajamarca, known as Conga, is partlyowned by local miner Buenaventura and wouldproduce between 580,000 and 680,000 ounces of gold annually.Conga would essentially replace the nearby Yanacocha mine run byNewmont and Buenaventura that is nearing the end of its life.

Protesters have expressed outrage that Humala gave the minerpermission a week ago to proceed with construction of theproject after Newmont agreed to comply with a more stringentenvironmental mitigation plan recommended by outside experts.

Newmont has agreed to build larger reservoirs that wouldreplace two or more in a string of alpine lakes and guaranteeyear-round water supplies in towns that suffer during the dryseason. It started work on those reservoirs over the weekendafter nearly all construction work one the mine had been stoppedsince November because of protests.

The company's local office said on Tuesday the violence wasunfortunate, but said it was "reaffirming its commitment toCajamarca."

Peru, which has vast mineral resources, is the world'ssecond-largest producer of copper and sixth of gold, but manymining communities suffer from widespread poverty and complain adecade-long economic boom has passed them by.